add tracing to saffron

[martyall]

I'm adding the tracing library in order to uniformly handle the combo of instrumenting and logging like you find here

Here is an example output from running a test locally:

2025-01-22T19:32:19.946Z DEBUG saffron: Encoding file input_file="/home/martyall/tmp/bigfile.txt"
2025-01-22T19:32:20.925Z  INFO encode: saffron::serialization: close time.busy=829ms time.idle=5.01µs
2025-01-22T19:32:21.089Z DEBUG saffron: Writing encoded blob to file output_file="/home/martyall/tmp/bigfile.bin"

2025-01-22T19:32:21.543Z DEBUG saffron: Decoding file input_file="/home/martyall/tmp/bigfile.bin"
2025-01-22T19:32:23.293Z  INFO decode: saffron::serialization: close time.busy=1.35s time.idle=4.30µs
2025-01-22T19:32:23.293Z DEBUG saffron: Writing decoded blob to file output_file="/home/martyall/tmp/bigfile-decodedtxt"

This is what it actually looks like. This was useful in optimizing the decode function in #2959 . There were many times I wish I had this when working on o1vm -- specifically the ability to have nested spans to help locate problems rather than manually tracing debug statements

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 22 lines in your changes missing coverage. Please review.

Project coverage is 76.79%. Comparing base (228e998) to head (70732bc).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
saffron/src/main.rs	0.00%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2961      +/-   ##
==========================================
+ Coverage   76.53%   76.79%   +0.26%     
==========================================
  Files         258      258              
  Lines       61160    61166       +6     
==========================================
+ Hits        46807    46971     +164     
+ Misses      14353    14195     -158     

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